1. Field of the Invention
The present invention relates to an ink-jet printhead. More particularly, the present invention relates to an apparatus for controlling a temperature of an ink-jet printhead that can rapidly control operating conditions of a heater driving field effect transistor (FET) according to a current flowing through the heater driving FET so that a temperature of a printhead substrate is increased to and maintained at a predetermined temperature.
2. Description of the Related Art
To achieve high printing quality, an ink-jet printer heats a printhead substrate to a predetermined temperature and maintains a size of ink droplets discharged from a nozzle of the printhead at a predetermined size. For improved, more stable printing quality, various apparatuses for controlling the temperature of an ink-jet printhead to improve response to a change in the temperature of the printhead substrate have been developed.
In one conventional method of controlling the temperature of a printhead, supplementary heaters, which are resistance heaters, heat a printhead substrate. A plurality of main heater driving transistors, however, which is connected in parallel to increase current flow, is disposed on a middle of the printhead substrate to supply sufficient current to a main heater. Therefore, in the method of controlling the temperature using separate supplementary heaters, the printhead substrate cannot be heated uniformly because the locations of the supplementary heaters are restricted to sides of the printhead substrate due to limited availability of space on the printhead substrate.
In addition, because a temperature sensor disposed around edges of the printhead substrate is located at a different location from the supplementary heaters, it is difficult to quickly control the supplementary heaters in response to the temperature sensed by the temperature sensor.
As an alternative, an apparatus for controlling the temperature of a printhead that heats a printhead substrate using only a main heater, i.e., without resistance heaters, has been suggested, as illustrated in FIG. 1.
Referring to FIG. 1, when the apparatus is in an ink discharging mode, a controller (not shown) drives a driving transistor 16, which has a larger capacity than a warming transistor 18, and allows sufficient current to flow through a main heater 14, which is a resistance heater, to discharge ink.
When the apparatus is in a substrate heating mode, the warming transistor 18, which operates each ink chamber 13 of a printhead, applies a warming pulse to the main heater 14 in response to a warming control signal received when the temperature of the printhead sensed by a temperature sensor (not shown) is lower than a predetermined temperature to maintain the temperature at the predetermined temperature.
Since the warming transistor 18 increases the temperature of the substrate using the main heater 14, a signal output from the warming transistor 18 to the main heater 14 must be limited to have a low enough voltage or a short enough signal pulse width so as not to discharge ink 10 via a nozzle 12. Therefore, it takes a significant amount of time to increase the temperature of the substrate to the predetermined temperature due to a low heating temperature of the main heater 14.
FIG. 2 is a circuit diagram of a conventional apparatus for controlling the temperature of a printhead that heats a printhead substrate using an on-resistance of a transistor. This method uses an operating resistance of a transistor and does not include a resistance heater as a supplementary heater.
Referring to FIG. 2, when a control signal Q1 applied to a gate of a first pass FET 200 is at a high level, a voltage source connected to a drain of the first pass FET 200 is supplied to a drain of a second pass FET 210, which includes a plurality of transistors, and a drain of an enable FET 220, via a source of the first pass FET 200. The drain voltage of the enable FET 220 is applied to a gate of a main heater driving FET 230, and when the gate voltage is at a high level, the current by a heater voltage flows to a ground via a main heater 240 and the main heater driving FET 230.
An on-resistance of each of the first pass, second pass, and enable FETs 200, 210, and 220 is 200 ohms, which is higher than a resistance of the heater driving FET 230. When the first pass, second pass, and enable FETs 200, 210, and 220 operate in response to control signals Q1 through Q5, and CE, respectively, applied to gates of the first pass, second pass, and enable FETs 200, 210, and 220, the first pass, second pass, and enable FETs 200, 210, and 220 are heated due to the on-resistance, and increase the temperature of a printhead substrate.
However, even in a substrate heating mode or a heater heating mode, the first pass FET 200 always remains “on” and increases the temperature of the printhead substrate, thereby making it difficult to control the temperature of the printhead substrate. In addition, the heater driving FET 230, which is composed of a plurality of transistors (not shown), takes up most of the area of the printhead substrate, and the heatable first and second pass FETs 200 and 210 are uniformly disposed, thereby making it difficult to control the temperature of the printhead substrate.